The example embodiments disclosed herein relate to semiconductor memory devices, and more particularly, to nonvolatile memory devices and methods of programming the same.
Semiconductor memory devices are classified into volatile semiconductor memory devices and nonvolatile semiconductor memory devices. The volatile semiconductor memory devices have a high read speed and a high write speed. However, the volatile semiconductor memory devices lose their stored data when their power supplies are interrupted. In contrast, the nonvolatile semiconductor memory devices retain their stored data even when their power supplies are interrupted. Thus, the nonvolatile semiconductor memory devices are used in memorizing data which should be kept regardless of whether a power supply is supplied or not.
Since flash memories, which are among the nonvolatile memory devices, have a function electrically erasing cell data in a lump, they are widely used in a computer and a memory card. Flash memories are classified into a NOR type and a NAND type according to a condition of a connection between cells and a bit line. A NOR type flash memory is a memory in which at least two cell transistors are connected to one bit line in parallel. The NOR type flash memory may store data using a channel hot electron method and erases data using a Fowler-Nordheim Tunneling method. In contrast, a NAND type flash memory is a memory in which at least two cell transistors are serially connected to one bit line. The NAND type flash memory may store program data and erases data using F-N tunneling method.
In NAND type flash memory device, the cells that are connected to a selected word line and not to be programmed may be soft-programmed by a program voltage applied to the selected word line, due to characteristics of the cell structure as widely known.
In program operation of NAND type flash memory, a high program voltage (e.g., 20V) is applied to a word line connected to selected memory cells. As a result of the program operation, a threshold voltage of the selected memory cells is changed to have a high level, or kept at a previous level according to data written in the memory cells. Memory cells, from among the selected memory cells, that keep a previous threshold voltage (referred to as a program-inhibited cell) experience a bias condition which is referred to as a program inhibit. Generally, the F-N tunneling phenomenon may be prevented in the selected memory cells that are not to be programmed by increasing the channel potential so as to reduce the strength of an electric field applied to a charge storage layer. The program-inhibited cell may be soft-programmed by a high program voltage due to characteristics of the cell structure of NAND flash memory. The soft-program is called a program disturbance. Also, a pass voltage (Vpass) is provided to unselected word lines. The pass voltage (Vpass) applied to unselected memory cells connected to a string that is be programmed may cause an unintended programming of unselected memory cells due to a potential difference between a channel voltage and the pass voltage. The unintended programming is called a pass disturbance.